The present invention relates to an apparatus and a method for cooling a hot rolled steel strip having a high temperature and a method for manufacturing the hot rolled steel strip.
In general, a hot rolled steel strip is manufactured in a step where a slab is heated to the specified temperature in a heating furnace and is rolled to the required thickness by a rough rolling mill to form a rough bar, and finally the resultant bar is rolled by a continuous hot rolling mill having plural rolling stands. The hot rolled steel strip is cooled at a cooling stand on a runout table and then is coiled by a coiler.
An online cooling apparatus to transfer as rolled high temperature steel strip and to continuously cool before coiling by the coiler should be first designed to consider steel strip transferring ability.
For example, for cooling an upper surface of the steel strip, circular laminar cooling nozzles can be provided at an upper area of the steel strip transfer roll (called a roller table) and at a straight line over the width of the steel strip for ejecting plural laminar cooling water. The runout table comprises plural transfer rolls.
At this time, laminar nozzles with the same length as an axial length of the transfer roll is arranged just above the roll to prevent a steel strip path line from lowering below a line connecting upper contact points of the transfer roll even when pressing the steel strip by water pressure of the falling down cooling water. In addition, spray nozzles are arranged between transfer rolls to eject cooling water upward for cooling the lower surface of the steel strip.
Therefore, this cooling mode does not ensure an exact symmetrical cooling for the upper and lower surface of the steel strip, resulting in intermittent cooling especially at the upper surface of the steel strip. This makes a rapid cooling (for example, cooling speed of 200xc2x0 C./sec or more for 3 mm in sheet thickness) impossible practically.
Recently, the rapid (strong) cooling, however, has been required to produce the hot rolled steel strip with fine grain size because of excellent machinability and to manufacture low Ceq high strength product.
Upon rapid cooling of the hot rolled steel strip, the conventional cooling apparatus has been involved in the following problems.
At rapid cooling, a cooling start point is different at the upper and lower surfaces of the steel strip, which causes to generate non-uniformity in material property. After cooling, cooling water remains at the upper surface of the steel strip to cause excessive cooling at the upper surface. The excessive cooling is not uniform in a longitudinal direction, resulting in variation in cooling finish temperature in this direction.
In the width direction, cooling water tends to flow from sides of the steel strip to both line sides to cause excessive cooling at the end compared with the center of the strip, fluctuating the temperature finish time. This makes material property non-uniform.
Hence, a water breaking method has been proposed such as a method to eject fluid in slant direction across the steel strip to discharge cooling water JP-A-9-141322, (the term xe2x80x9cJP-Axe2x80x9d referred to herein signifies xe2x80x9cUnexamined Japanese Patent Publicationxe2x80x9d) or a method using a restriction roll (called a pinch roll) as a water block roll to interrupt cooling water, JP-A-10-166023.
However, the former method when applying strong cooling is useless because a large amount of cooling water remains on the steel strip. In the latter method, a top of the steel strip is left at a free state during transfer at the interval from the final rolling mill to the coiler, the strip passes at non-restrained state moving up and down in waving action.
As a result, the restriction roll if provided at the roller table disturbs safe passing of the strip, which is difficult to apply the roll as the cooling apparatus for the runout. Strong cooling if applied at the top of the vibrating steel strip at non-restricted state will further escalate vibration of the top end of the steel strip unavoidably to damage due to contact with the restriction roll.
On the other hand, JP-A-6-328117 proposes an effective cooling method by increasing cooling water at the steel strip top end for the lower surface than that for the upper surface. Change in the cooling water ratio, however, will unbalance the cooling effect to upper and lower surfaces especially to make unavoidably material property non-uniform. In addition, the strong cooling necessary for changing in material property is difficult because of insufficient cooling at the lower surface.
In particular, for cooling so called thinner sheet less than 2 mm in thickness, the steel strip top vibrates up and down by cooling water pressure or the steel strip tends to fold at the last half of the runout table to disturb stable passing, finally stopping the steel strip passage.
In JP-B-59-50420, (the term xe2x80x9cJP-Bxe2x80x9d referred to herein signifies xe2x80x9cExamined Japanese Patent Publicationxe2x80x9d) a cooling water guide is arranged between plural roller tables in the frame provided in the feeding direction of the steel strip. To maintain the specified interval between the guide and steel strip surface, a press machine for the steel strip is disclosed by installing a guide roll at the guide.
This machine, however, is difficult to hold uniform interval between the cooling water guide and the steel strip surface because the steel strip top is transferred waving up and down. This method if applied for a thinner steel strip causes sticking trouble because of disturbing smooth passage at touching the steel strip top to the transfer roll.
The steel strip usually is not flat with an edge waving or center buckling. Such steel strip failed in its shape cannot be pressed by the guide roll, resulting in another leveler provided for flat shape to escalate working man-hour.
JP-B-4-11608, discloses a direct cooling apparatus to cool the steel strip just after delivering from the roll mill. But this apparatus is not available for installing a detecting sensor for steel strip temperature and sheet thickness during rolling step as significant items in quality control of the steel strip.
This requires an air cooling space after the final finishing mill to install a thermometer or a thickness gage at the space. However, cooling is difficult to start at the steel strip top end, because it vibrates up and down at free state.
While, JP-U-57-82407 discloses a technique giving a travel driving force to the steel strip by providing another driving roll which can rotates upwards to the table roll.
This technique, however, should arrange an upper driving roll as densely as the lower table roll. If not, the steel strip top end might be crashed into the roll clearance or be broken at the half way the steel strip top end once crashed into the upper or lower rolls generates up and down vibration due to reaction force to disturb stable passage, especially for thinner strip. Rolls if arranged densely at both upper and lower sides will disturb strong cooling because the cooling nozzle area is narrowed.
It is an object of the first invention to provide an apparatus and a method for cooling a hot rolled steel strip wherein the steel strip having no tension is cooled stably and strongly at a runout table arranged between a finishing mill and a coiler.
It is an object of the second invention to provide an apparatus and a method for cooling a hot rolled steel strip wherein cooling water is removed rapidly from the surface of the steel strip during cooling the steel strip, to move the steel strip smoothly and to produce the hot rolled steel strip without any defect.
It is an object of the third invention to provide an apparatus and a method for cooling a hot rolled steel strip wherein a top end of a steel strip moves smoothly from a final finishing mill to a coiler to cool the steel strip rapidly and to ensure a cooling efficiency.
It is an object of the fourth invention to provide a method for manufacturing a hot rolled steel strip with a cooling step of cooling a hot rolled steel strip. The cooling step uses either of the cooling apparatus and cooling methods according to the first through third inventions.
The first invention is to install a lower surface cooling box between transfer rolls on the runout transferring the steel strip, and to provide an upper surface cooling box movable vertically to corresponding positions to the lower surface cooling box for symmetrical water ejection to the steel strip in upper and lower directions, and to pass the steel strip to the center of a confluence of the cooling water, and to provide a water breaking roll rotating in synchronization with the peripheral speed of the transfer roll, and to lower rotating the water breaking roll concurrently with passing the cooling apparatus, and to lower the upper surface cooling box at the same time to cool the steel strip.
In addition, the first invention provides the cooling apparatus of the hot rolled steel strip to pinch the upper and lower surfaces at the top by the water breaking roll and the transfer roll concurrently with passage of the top end of the steel strip and concurrently to eject the cooling water at the following conditions from upper and lower surfaces of the steel strip and its cooling method.
Use of the cooling apparatus and cooling method enables to rapidly cool symmetrically the upper and lower surfaces and to manufacture stably the hot rolled steel strip with fine grain size by this online cooling.
This prevents excessive cooling without cooling water remaining on the steel strip at the downstream of the cooling apparatus, stabilizes the cooling stop temperature in both width and longitudinal directions of the steel strip, equalizes completely cooling conditions at both upper and lower surfaces, eliminates to occur bending during cooling and residual stress after cooling, and manufactures stably the uniform hot rolled steel strip with a constant grain size in the longitudinal and width directions.
This also enables to eject the cooling water at the same cooling condition as the center of the steel strip under tension even under non-tension before coiling the steel strip top by the coiler, resulting in uniform material property in upper and lower surfaces as well as the longitudinal direction to raise a product yield rate to stabilized the quality of the steel strip.
The second invention is intended to solve these problems to arrange a water breaking means just above the transfer roll at an entrance, exit, or entrance and exit sides at the cooling apparatus in the runout transferring the steel strip on plural rotating transfer rolls and in parallel with the transfer roll to install the water breaking means at the position where the steel strip and clearance exist.
The water breaking means can freely elevate up and down to employ a water breaking roll as a water breaking means with a preferable distance 1 to 10 mm between the water breaking roll and the steel strip to rotate the water breaking roll at the peripheral speed of the water breaking roll roughly to coincide with the transfer speed of the steel strip, and to install at least one or more fluid ejection nozzles at an opposite side of the cooling apparatus to discharge rapidly the cooling water flown from the clearance between the water breaking roll and the steel strip away from the steel strip.
The Invention provides a structure not to damage or disturb passage of the product by evacuating the roll upwards at passing the steel strip top. The water breaking roll effectively discharges the cooling water from the upper surface of the steel strip on the runout after rolling.
As a water breaking means, the water breaking roll is the best choice, but another water breaking means with a baffle installed at a proper angle can also be acceptable.
An upper and lower cooling boxes comprising the cooling apparatus are arranged at a position facing each other across the steel strip to be transferred to eject the cooling water to the hot rolled steel strip. The upper cooling box elevated freely to the transfer roll is equipped with the water breaking roll at least at its exit side and at a position facing to the transfer roll.
A distance between a nozzle outlet discharging cooling water as a laminar flow and the hot rolled steel strip is ranged to 30 to 100 mm.
Use of above cooling apparatus and the cooling method enables to effectively discharge the cooling water from upper surface of the steel strip to manufacture stably the hot rolled steel strip with a fine grain size.
The third invention is intended to solve these problems to provide an accompanying roll continuously from the finishing mill side with a clearance over sheet thickness of the steel strip just above the transfer roll in the runout transferring the steel strip on the transfer means comprising the plural rotating transfer rolls behind the final finishing mill to rotate the accompanying roll nearly at the same peripheral speed as the transfer roll to push out the steel strip backwards by rotating at higher speed than the transfer speed of the steel strip.
In addition, a plate passing guide is provided between transfer rolls and between accompanying rolls to pass the steel strip between the guides. A cooling nozzle is installed at an opposite side of the steel strip to the guide to eject the cooling water from upper and lower sides of the steel strip for cooling. Such cooling apparatus is installed behind the final finishing roll and in the runout in front of the coiler.
Furthermore, at least one or more pinch roll pairs to pinch steel strip at the position during plate passage or just after the cooling apparatus to reach the steel strip top end to the pinch rolls pair giving tension to the steel strip at an upstream side to stabilize the plate passing. A rotating contact of the pinch roll pair is released sequentially upon reaching the downstream pinch roll pair or coiler.
Use of the cooling apparatus and cooling method of the hot rolled steel strip can stably and rapidly cool the steel strip just after the roll mill. In particular, the same cooling condition as the center of the steel strip under tension is available even under non-tension before reaching coiler, resulting in completely equal cooling condition to upper and lower surfaces at the steel strip top.
Restraining occurrence of bend or residual stress after cooling can produce uniform grain size in longitudinal and width directions. This results in uniform product with a high yield rate to supply the hot rolled steel strip with stabilized quality.
This cooling apparatus and cooling method ensures a constant path line of the steel strip using a fluid pressure to prevent defect from occurring without any folding of the steel strip or deforming to an accordion like shape.
The fourth invention uses either of a cooling apparatus or a cooling method of the hot rolled steel strip according to the first through the third inventions to provide the cooling step for hot rolled steel strip cooling and to manufacture the hot rolled steel strip.
This results in an effective discharging the cooling water from upper surface of the steel strip not only to prevent excessive cooling to eliminate bending during cooling and residual stress after cooling but also to manufacture stably the hot rolled steel strip with uniform grain size In longitudinal and width directions.